This invention relates to a process for adjusting the electromagnetic radiation output from a solid-state radiation emitter.
Light emitting diodes (LED's), and similar devices such as liquid crystal shutters, are used, inter alia, to print on light-sensitive media such as photographic film, thermal imaging media such as those described in International Patent Application No. PCT/US87/03249 (International Publication Number WO 88/04237), and photoconductive receptors used in electrophotography. Typically, the LED's are provided in a long linear array.
To secure a high quality image, it is important that all the LED's have the same output under the same drive current and other conditions, since even small variations in output among the LED's may produce noticeable dark or light lines across the printed image. Unfortunately, the present state of technology does not allow for the fabrication of large LED arrays with sufficiently uniform output from each diode for imaging purposes, while still maintaining an acceptable yield of arrays from the semiconductor wafers on which the LED's are fabricated. Accordingly, various techniques have been devised to "correct" the output from the LED's as fabricated in order to provide an array of LED's with the uniform output required for imaging.
For example, according to U.S. Pat. No. 4,927,778 to Abbas, one of the major causes of non-uniformity in LED arrays as fabricated is the presence of parallel linear crystal defects in the wafer, which defects result in the production of dark line defects in the completed diodes. To deal with these dark line defects, this patent proposes examining each semiconductor wafer in advance of formation of diode arrays thereon to determine the nature and alignment of the parallel linear crystal irregularities, forming the diode arrays on the semiconductor wafer with their major axes parallel to the linear crystal irregularities, and subdividing the wafer to produce a plurality of chips each containing at least one of the diode arrays. This process requires detailed examination of each wafer prior to fabrication, and does not allow correction of point defects in the wafer which are not due to linear crystal defects.
Electronic techniques have also been used to secure uniform output from LED arrays. Such techniques rely upon adjusting the potential difference or drive current to each individual LED in accordance with a prior measurement of the variation of the output of each individual LED with the parameter being adjusted. For example, U.S. Pat. No. 4,878,222 to Lawrence describes a method of electrically modulating the light intensity emitted from semiconductor diode lasers by means of low voltage, low current modulating control bias signals. Also, U.S. Pat. No. 3,996,526 to d'Auria et al. describes an electronic method for controlling the output of an LED, in which a photodiode is located close to the radiating surface of the LED for picking off a part of the optical radiation and for delivering a feedback current which is used in a current negative feedback loop. However, such electronic output control techniques require the provision cf a separate control circuit for each emitter, and thus add greatly to the cost of complexity of multiple emitter devices, especially since LED arrays used in practice may contain from several dozen to several hundred individual LED's. In addition, such electronic techniques degrade the grey-scaling factors used to determine the desired output from each LED, and may thus adversely affect image quality when the image has a large number of grey levels, for example 64 or 256.
It is also known to apply coatings to the emitting surfaces of LED's for various purposes. For example, U.S. Pat. No. 4,840,922 to Kobayashi et al. describes a masked semiconductor laser which produces a very narrow beam intended for use in writing optical disks and similar applications. A masking layer is deposited over the light-emitting surface of the laser and then the laser is driven at high power to "burn" a small central aperture through the masking layer.
Similarly, U.S. Pat. No. 4,280,107 to Scifres et al. describes apertured and unapertured reflector structures for LED's and semiconductor lasers. These structures comprise, outwardly from the LED or laser, a layer of low refractive index, a layer of intermediate refractive index and a layer of high refractive index. Ablative means remove and form an aperture in the outer high index layer at the region of optical radiation emission from the device so that the level of reflectivity is highest at the center of the aperture, so that fundamental mode stabilization may be achieved.
U.S. Pat. No. 3,843,401 to Carroll describes a masking technique for semiconductor lasers which is somewhat similar to that described in the aforementioned U.S. Pat. No. 4,280,107. In Pat. No. 3,843,401, an anti-reflection coating is applied to the emitting face of a semi-conductor laser and a film of bismuth is applied over the anti-reflection coating. The laser is then pulsed at high power, and light absorbed by the bismuth causes an aperture to be formed in the bismuth layer over the active region of the laser.
There is thus a need for a simple process for adjusting the output of electromagnetic radiation from an emitter which does not require the provision of additional electronics and which can readily be applied to ensure that the outputs from a plurality of emitters in an array are substantially equal to one another, or differ from one another in accordance with a predetermined pattern. This invention provides such a process.